Variable length array in the middle of struct - why this C code is valid for gcc

Question

There is some strange code using VLA (Variable Length Arrays) which is treated as Valid C (C99, C11) by gcc 4.6:

$ cat a.c
int main(int argc,char**argv)
{
           


        

Answer 1

The authors of the C89 Standard recognized that many implementations implemented useful features which might be impractical on other implementations, and recognized that as a good thing. The Standard was intended as a minimum set of requirements for implementations; it was never intended to discourage implementations from providing features beyond that.

The Standard requires that if a conforming implementation allows a variable-length array to be declared within a structure defined at block scope, it must either document such behavior as an extension or issue a diagnostic when code contains such a declaration. Since an implementation would be free to process the code however it likes after issuing such a diagnostic, whether or not it documents an extension, the requirement to document extensions can only be meaningfully applied to extensions which do not generate diagnostics. That in turn would suggest that such things must be allowable.

The Standard does require that extensions not adversely affect the behavior of any Strictly Conforming programs, but since no such program could contain a VLA declaration within a structure that requirement is not a problem here.